Phosphors that can be efficiently excited with ultraviolet, blue or green primary radiation and have efficient emission in the blue, green, yellow, red or deep red spectral region are of very great interest for the production of white and colored conversion LEDs. Conversion LEDs are used for many applications, for example for general lighting, display backlighting, signage, display panels, in automobiles and in numerous further consumer products. Conversion LEDs for the backlighting of display elements, for example displays, differ significantly from conversion LEDs for general lighting. The demands on conversion LEDs for general lighting are especially a high light yield combined with a high efficiency, a high color rendering index and specific color temperatures (e.g. below 3500 K for what is called warm white light or, for example, 6500 K for what is called cold white light). For conversion LEDs for backlighting of display elements, particularly phosphors having narrowband emissions in the blue, green and red spectral region are required in order to cover a color space of maximum breadth. Moreover, there is great demand for colored conversion LEDs that render colors matched to consumer wishes (called “color on demand” applications).
Existing white-emitting conversion LEDs for general lighting and backlighting use a semiconductor chip that emits blue primary radiation and a red and green phosphor. A disadvantage of this solution is that the epitaxially grown semiconductor chips, based, for example, on GaN or InGaN, can have variations in the peak wavelength of the primary radiation emitted. This leads to variations in the white overall radiation, such as a change in the color locus and the color rendering, since the primary radiation contributes the blue component to the overall radiation. This is problematic particularly in the case of use of multiple semiconductor chips in one device.
In order to prevent variations, the semiconductor chips are sorted in accordance with their color loci (“binning”). The narrower the tolerances set with regard to the wavelength of the primary radiation emitted, the higher the quality of conversion LEDs consisting of more than one semiconductor chip. But even after sorting with narrow tolerances, the peak wavelength of the semiconductor chips can change significantly at variable operating temperatures and forward currents. In general lighting and other applications, this can lead to a change in the optical properties, such as the color locus and color temperature.
In the backlighting of display elements, such as displays in televisions, computer monitors, tablets and smartphones, manufacturers are trying to render the colors in a lively and true-to-life manner, since this is very attractive to consumers. For the backlighting of display elements, therefore, light sources with very narrow-band emissions, i.e. a small half-height width, in the green, blue and red spectral region are required to cover a color space of maximum breadth. As light sources for backlighting applications, it is predominantly the case that a blue-emitting semiconductor chip is combined with a phosphor having a peak wavelength in the green and a phosphor having a peak wavelength in the red spectral region.
Conversion LEDs for backlighting applications conventionally use, as green phosphor, for example, an yttrium aluminum garnet, a lutetium aluminum garnet or a β-SiAlON (Si6−zAlzOzN8−z:RE or Si6−xAlzOyN8−y:REz with RE=rare earth metal). However, yttrium aluminum garnet has an emission peak having a large half-height width, and so the achievable color space is restricted by considerable filter losses and the efficiency is also lowered. β-SiAlON, with a half-height width of below 60 nm, has narrow-band emission in the green spectral region that leads to more saturated green rendering than with a garnet phosphor. However, the β-SiAlONes lack good internal and external quantum efficiency, which makes the overall backlighting comparatively inefficient. Furthermore, the production of these phosphors requires very high temperatures and complex equipment. Thus, the phosphor is very costly to produce, and hence so are the conversion LEDs including this phosphor.
Quantum dots, owing to their very narrow-band emission, are also used for conversion of primary radiation for backlighting applications. However, quantum dots are very unstable.
Moreover, most commercially available quantum dots include harmful elements such as Hg or Cd, the concentration of which in commercial electrical and electronic devices is limited under the RoHS regulations (“reduction of hazardous substances”, EU Directive 2011/65/EU).
Known blue-green to green phosphors for conversion LEDs are, for example, the phosphors Ca8Mg(SiO4)4Cl2:Eu, (Sr, Ba)2SiO4:Eu and Lu3(Al,Ga)5O12:Ce. However, conversion LEDs including these phosphors have inadequate color purity and cannot attain particular color loci, and they are therefore not an option for many “color on demand” applications.
Lighting devices, for example white light-emitting diodes, can be used as backlighting. For this purpose, red-emitting phosphors are generally needed. However, the use of red-emitting phosphors in lighting devices is limited to a few phosphors, for example to nitridosilicate phosphors, for example (Ca, Sr, Ba)2Si5Ne:Eu, and nitridoalumosilicate phosphors, for example (Ca,Sr)AlSiN3:Eu. However, these phosphors show a number of disadvantages in relation to color space coverage, spectral position, half-height width (FWHM) and the overlap region with conventional filter units for backlighting. In the case of use of (Ca,Sr,Ba)2Si5N8:Eu, the shift in the emission wavelength from the orange to the red spectral region can be effected by substitution of barium for strontium and/or calcium. The result is a phosphor, but one that is very unstable. In addition, phosphors having a high dominant wavelength (λdom) with values of more than 605 nm have a very high half-height width. This leads to a low-efficiency phosphor and low color saturation. Nitridoalumosilicate phosphors typically show a dominant wavelength of up to 608 nm, but a broad emission spectrum and hence low luminescence efficiency.
Phosphors that emit in the red spectral region and have an emission spectrum having a small half-height width are therefore of great interest, especially in the case of use in lighting devices, for example backlighting.